The idea of anchoring transition metal catalysts to organic polymers has interested scientists, wherein bound catalyst studies have been made to produce heterogenized catalyst systems having the distinguishing characteristic of being easily separable from the reaction media (Hodge, P., Sherrington, D. C. (Eds.), Synthesis and separations using functional polymers, John Wiley & Sons, New York, (1988)).
Transition metal polymer-bound catalysts may offer a number of advantages, such as
greater catalytic activity, due to lack of formation of ligand-bridged complexes as one catalytic site may be isolated from another,
two active catalysts anchored to the same polymer backbone can in some cases be used to conduct sequential multistep organic synthesis,
minimized losses during use of polymeric catalysts,
increase of substrate selectivity due to an increase in the steric environment. Further, the selectivity of the polymer towards different substrates can be controlled by the loading of the catalyst on the polymer support, controlling the degree of resin swelling and introducing optically active groups in the polymer around the active site.
A polymer support used in a particular application should fulfill a number of important functions simultaneously. First, the support must possess the correct mechanical properties. For example, in column or batch applications, resin beads must be mechanically strong enough compression and fractional friction. Secondly, the support must possess the correct physical structure in order to ensure that a high amount of the functional groups in the material are accessible to the reaction phase. Finally, the support must provide the correct microenvironment to optimize the process being carried out, e.g. it must provide the correct polarity, hydrophilicity, microviscosity etc. In general, these support requirements have been demanded by default rather than by careful argument and design. When looking at the traditional styrene divinylbenzene resins it seems clear that the desirable features, especially in the accessibility-capacity relation, tend to be mutually exclusive and that for any particular application a compromise had to be made (Guyot, A., Reactive Polymers. 16 (1992), 233).
Further, since polymer supported catalysts are more expensive than their homogeneous analogues, it is vital that they can be recycled.
A useful method for preparing polymer bound reactants, with a potential of solving many of the problems mentioned above, is grafting, and especially radiation grafting, which offers promising new opportunities (Hartley, F. R., J. Polym. Sci., Polym. Chem. Ed., 20, (1982), 2395; Garnett, J. L., J. Polym. Sci., Polym. Lett., 19, (1981), 23; Akelah, A., J. Appl. Polym. Sci., 28, (1983), 3137). Radiation grafting involves taking a polymer with appropriate morphology and physical properties and introducing reactive sites, free radicals, into the polymer chain by irradiation. The free radicals can either combine to give cross-links, as is the case for example polyethylene, or cause chain scission, as is the case for polypropylene. In the presence of vinyl monomers, on the other hand, the free radicals can initiate graft copolymerization.
The preparation of graft copolymers and the use of graft copolymers in a variety of applications are well known in both literature and patents (Stannet, V. et. al., Radiat. Phys. Chem., 35, (1990)).
Three different methods of radiation grafting have been developed and most of the work done has concentrated on the use of low dose rate gamma rays from .sup.60 Co sources. During the past few years, however, there has been much interest in using high energy electrons from accelerators with high dose rates (10.sup.6 -10.sup.9 rads/sec), since these high dose rates make radiation chemical processes commercially more attractive. The chemistry involved is, however, similar whether gamma or electron radiation is utilized, and therefore the graft result using the different sources does not significantly differ. The three methods of radiation grafting that have received special attention are: (1) direct radiation grafting of a vinyl monomer onto a polymer (mutual grafting), (2) grafting on radiation-peroxidized polymers (peroxide grafting) and (3) grafting initiated by trapped radicals (pre-irradiation grafting).
Mutual grafting by irradiating of the polymer in the presence of the monomer is a fairly simple and effective method, since the free radicals initiate polymerization immediately upon generation. The disadvantage of this method is, however, that simultaneously with the graft copolymerization, homopolymerization of the monomer occurs upon irradiation.
When grafting on radiation-peroxidized polymers, the polymer is first irradiated in the presence of oxygen, thus forming peroxides and hydroperoxides that are stable and can be stored in the polymer for a long period of time. Grafting is activated by cleavage of the peroxides or hydroperoxides by heat, UV-light or catalysts in a monomer solution.
Pre-irradiation grafting by irradiation of the polymer alone in an inert atmosphere and immersing the irradiated polymer in a monomer solution requires additional steps in comparison to direct radiation or mutual grafting, but the advantage is that only a small amount of homopolymer is formed, mainly by a chain transfer process. The pre-irradiation grafting process is controlled by the diffusion of the monomer in the polymer and can to some extent be facilitated by the use of solvents that are able to swell the formed graft copolymer.
Pre-irradiation grafting is mostly preferred since this method produces only small amounts of homopolymer in comparison to mutual grafting.